Document internal namespace

Eigen/Core

diff --git a/Eigen/Core b/Eigen/Core
index 16e388f..3f1efb5 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -220,6 +220,9 @@
 /** \brief Namespace containing all symbols from the %Eigen library. */
 namespace Eigen {
 
+/** \brief Namespace containing low-level routines from the %Eigen library. */
+namespace internal {}
+
 inline static const char *SimdInstructionSetsInUse(void) {
 #if defined(EIGEN_VECTORIZE_AVX)
   return "AVX SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2";

unsupported/Eigen/doxygen.hpp

diff --git a/unsupported/Eigen/doxygen.hpp b/unsupported/Eigen/doxygen.hpp
new file mode 100644
index 0000000..0314885
--- /dev/null
+++ b/unsupported/Eigen/doxygen.hpp
@@ -0,0 +1,19 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2014 Chen-Pang He <jdh8@ms63.hinet.net>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// As its name suggests, this file is intended to be read by Doxygen.
+// There is nothing defined so we need no #include guard.
+
+//! \brief Namespace containing all symbols from the %Eigen library.
+namespace Eigen {
+
+//! \brief Namespace containing low-level routines from the %Eigen library.
+namespace internal {}
+
+}